Dual 4K slowness due to xorg threading concept?

[Christopher Barry]

On Fri, 02 Oct 2015 09:44:19 +0200
Thomas Wunder <foren at thomas-wunder.de> wrote:

>Hi all,
>
>Recently I got a new workstation consisting of these components:
>Two 4K Monitors (DP 1.2 at 60Hz), i7-5820K (not yet overclocked), MSI
>X99A- based mainboard, Radeon R7 250.
>This is the software I'm running on it (Debian Jessie):
>Linux 3.16.0 Debian build 4, fglrx 14.9, xorg 7.7, (KDE 4.12)
>
>Problem:
>"Graphics stuff" seems to work smoothly on lower resolutions (e.g. 2x 
>2560x1600). On high resolution (2x 3840x2160) still everything 
>(moving/resizing windows, playing videos, scrolling through lists,
>text views or consoles, opening menus) goes smoothly but only as long
>as single windows are really small (and as long as there are not too
>many of them). As soon as a window becomes half the size of a screen
>or even full screen, operations mentioned before become horribly slow
>(K-Menu takes up to 8 secs to even open up) and CPU usage of the Xorg
>process is vastly increasing (to about 100%). I don't really have an
>idea of what's going wrong here. There are no apparent errors etc.
>The only thing that seems really suspect to me is the 100% CPU which I
>would attribute to a (multi-)threading issue... I'd really appreciate
>if you could give me a hand and point me to the right direction to get
>things working.
>
>Cheers
> Thomas

http://gpu.userbenchmark.com/Compare/AMD-R9-380-vs-AMD-R7-250/3482vsm8217

Get a real card would be my advice.